This application relates to the art of motor control circuits and, more particularly, to control circuits for motor start windings. The invention is particularly applicable for use with capacitor start motors and will be described with specific reference thereto. However, it will be appreciated that the invention has broader aspects and can be adapted for use with other motors such as split phase induction motors.
A motor start winding is activated to start a motor and is deactivated when the motor reaches operating speed. The motor then continues to operate on the main or run winding, and the start winding is reactivated if additional motor torque is needed for a motor load that causes a significant reduction in motor speed. The start winding may be reactivated and deactivated frequently during normal operation of the motor.
Historically, activation and deactivation of the start winding has been accomplished with centrifugal switches and mechanical relays. Replacement of such devices with electronic controls provides improved life and reliability, along with reduced size and weight. However, it is difficult to maintain reasonable accuracy of the motor rpm trip points at which the start winding is activated and deactivated because the motor current varies with variations in line voltage and motor winding temperature. It would be desirable to have an electronic control arrangement that compensates for such variations and provides improved accuracy of the motor rpm trip points.
It is sometimes desirable to adjust the motor speeds at which the start winding is deactivated and reactivated. Adjustment of centrifugal switches to change the motor rpm trip points is cumbersome and difficult to accomplish with reasonable precision because it concerns adjustment of movable parts and spring tension. It would be desirable to have an electronic circuit that provides a simplified and more accurate way of adjusting the motor rpm trip points.